1. Field of the Invention
The present invention relates to the fork tree used to connect the fork of a motrocycle to the motorcycle body steering head.
2. Description of the Prior Art
An important consideration in the design of the front fork assembly of a motorcycle is the degree of "trail" exhibited by the fork. The trail can be defined as the horizontal distance by which a vertical plane passing through the front wheel axis W trails the point of intersection of the steering axis A with the ground. The trail is shown and designated as "T" in FIG. 1.
In order to optimize the riding characteristics of the motorcycle, the size of the trail must be correctly set. If the trail is too large, the motorcycle is stable at high speed, but is oversteered and hard to balance at low speeds and in curves. If, on the other hand, the trail is too small, or is negative, the motorcycle is easy to steer at low speeds, but has no self steering properties at high speed, and may wobble. Generally, with a correct trail value of between 5 and 15 cm, the motorcycle is easy to handle at low speed and follows curves with no wobble at high seed.
However, in conventional motorcycles the precise setting of the trail is difficult to set, since it depends upon many factors such as the length of the fork tubes and the point of balance of the motorcycle. This is particular true for "chopper" type motorcycles having long fork tubes.
It has been knwon to adjust the trail value of motorcycle by adjusting the spacing beteen diferent elements of the steering structure. This has been conventionally performed in one of two ways. One known way has been to form the adjustment of the parts in steps, followed by inspection after each step. However, since the distances between steps have been long, it has not been possible to obtain a correct trail value, especially in choppers having long fork tubes. Conventionally, the motorcycle fork tubes are connected to the steering head H by a fork tree assembly consisting of a single part. In such conventional arrangements, an attempt to modify the trail value was performed by extending the part corresponding to that having the hole 6a in FIG. 2 rearward (downward as seen in FIG. 3) in providing this part with a second hole parallel to the hole 6a. By inserting a pin in the second hole, the distance between the parts was increased and thereby the trail was changed.
Another known way to change the trail value was to perform a stepless infinitely variable adjustment. This technique is shown in U.S. Pat. No. 3,866,946 and U.S. Pat. No. 4,082,307. However, it was not known to perform such a stepless adjustment in a fork tree of the type used in the present invention, nor was known to perform the trail adjustment by means of the elements of the present invention.